Ivy League rockers Vampire Weekend have always been a unique musical math equation: One part house-party pop and two parts sonic safari, multiplied by Cape Cod plus keyboards, all squared by the new millennium.

The New York foursome took the stage at the Roseland Ballroom as part of American Express’ “Unstaged” series last night – its past alumni include Jack White, the Killers, and Coldplay, matched up with directors like Wernor Herzog and Gary Oldman; watch previous clips here – on the final night of the Tribeca Film Festival. And with the set’s turned-up bass drum and synchronized lights, the Roseland suddenly felt less like a rock venue than full-on dance hall.

The show had one special unannounced guest: SNL and Portlandia star Fred Armisen, opening under a British accent as “Ian Rubbish” and singing songs about boots, policemen, and Maggie Thatcher. Everything else, though, was Vampire: the band played for almost 90 minutes — a longer running time than their two albums-to-date, and then some — avoiding most hints of a pause.

No “Taxi Cab,” no “I Think Ur a Contra.” All “Campus” and “Horchata” and “Cousins,” with Chris Tomson’s drums in the spotlight in a way they rarely are in the studio. Rostam Batmanglij’s keys, always the melodies’ secret sauce, sounded happy to support the thump. (A quibble: sound levels, y’all. Some things got swallowed.) Even “Cape Cod Kwassa Kwassa” came out sounding like a ground stomper.

The evening was designed all in white, with four floating columns and a grand gold mirror facing the audience, which would flicker with images. The stage was decked out with lights, too — reds and blues and greens — which rattled like machine guns to the beat. As the band shifted into the final songs of the night, the giant white curtain stretched behind everything else fell away: the real background was a crazy, seasick wallpaper-style oversized floral.

Frontman Ezra Koenig kept reminding people to move, but that’s just probably because he couldn’t really see the crowd (or beside him, where bassist Chris Baio was happy not to keep still). Tomson kept breaking it down like Phil Collins. At one point, the band set into “I Stand Corrected,” slowed almost to a standstill, and one fan raised a flame. A few others sent puffs of not-cigarette-smoke into the air. The guy in front of me danced in a safari jacket, collar popped, as is the style.

There was time for new material, too, and VW faithfully cued up the four songs-so-far from its next album, Modern Vampires of the City. The best of them is still the sun-blasted “Unbelievers,” though the Wham!-y “Diane Young” wonderfully molds to Koenig’s gymnastic voice. The group also debuted two new songs: “Everlasting Arms” (a feel-good wish with a Paul Simon beat) and the positively lonesome “Obvious Bicycle.”

Including the concrete-and-keys rue of “Step” and “Ya Hey,” we’ve now heard six tracks from an album that has 12 total, so we’re halfway there. Very early prediction: Modern Vampires will be Vampire Weekend’s best album. Corollary: It will sound only something like other Vampire Weekend albums. The self-regard has been boiled away. Not everything is slower, but it has the feeling of slowness, of a purpose of feeling that cuts through the flashy insistence of, say, “One (Blake’s Got a New Face).” If the band has always been playing chamber pop shot through a kaleidoscope (I’m doing it again!), then we should expect some new colors.

Director Steve Buscemi took the stage for a brief moment halfway through the concert, leaving the audience both excited and confused. (Steve Buscemi? Steve Buscemi!) Koenig explained that Buscemi was responsible for the composition of the show — the lights and set design, probably, and at least the possible suggestion to bump up the beat. But the actor-director only stuck around long enough to sing a quick bar. He didn’t take a bow before leaving, but he should have.